The present invention relates to a catalyst for the heterogeneous catalyzed gas phase oxidation of propene into acrolein which is to be used in tube bundle reactors.
The strongly exothermal mixing of propene on heterogeneous catalyst with a oxygen-containing gas leads not only to the desired acrolein product but also to a series of undesirable by-products. It is known that by means of good dissipation of reaction heat, for example in tube bundle reactors, one can prevent the local overheating of the catalyst and the attendant increased formation of by-products.
It is furthermore known that by means of the size and outer shape of the catalyst body, one can influence the pressure loss of a catalyst charge. The internal structure of the catalyst body (porosity, length of diffusion paths) critically determines the mass transfer and heat transfer in the catalyst and thus, has a major bearing on selectivity along with the composition of the catalytically active mass.
High compression strength and abrasion resistance are basic pre-requisites if one is to be able to use a catalyst in industrial practice. High abrasion would lead to a situation where, during the filling of the tubes of a tube bundle reactor, the scattering of the pressure losses of the individual tubes would be high, which results in different flow through rates along with impairment of selectivity.
German Patent 31 25 061 describes a method for making acrolein, using shell catalysts. In the case of a shell catalyst, one can avoid the local overheating as a result of the temperature equalizing effect of the inert carrier; the diffusion paths for the gaseous reagents are relatively short in the thin shell.
German published patent application 33 38 380 describes ring-shaped or hollowed cylinder-shaped catalysts for the oxidation of propene into acrolein. These catalysts are made from a mass containing Mo, Fe, Bi and W. These catalysts can be considered as being derived from shell catalysts in that the inert core of the shell catalyst is replaced by an "inert cavity", while the shell is opened at two opposite points for access of the reagents to the cavity. Compared to the shell catalysts, these ring-like or hollowed cylinder-like catalysts reveal an enlarged ratio between the outer surface and the volume. As a result, the active mass is more accessible to the reagents. However, the low pressure loss and the high heat dissipation of the shell catalysts are present in these catalysts as well. To obtain sufficient mechanical strength in the case of the "hollowed catalyst", the active mass is highly compressed which results in the internal structure being adversely affected.
Ring-shaped catalysts, which have rounded end faces for the purpose of improving the filling capacity, are also described in European published patent application 0 184 790. Although neither the catalyst mass, nor a special production method are indicated; in particular no measures are illustrated for the attainment of a particularly favorable internal structure.
In order to make optimum use of active mass, the internal structure of the catalyst must be made so that a possible high reaction velocity will not be limited by an obstruction to mass transfer within the catalyst. European published patent application 0 279 374 shows an experiment aimed in this direction. A production method is described for a catalyst containing Mo, Fe and Bi, characterized by specific surface, pore volume, and pore distribution. Depending on the process used, however, one can obtain only catalyst particles of approximately spherical shape, that is to say, with a small ratio between surface and volume, or the particles would have to become very small. But, when it comes to industrial use, there are limits on that, due to the attendant high pressure loss.
Catalysts made and used according to the known state of the art present some disadvantages as regards the described aspects. By using differently shaped bodies, one either tries to shorten the diffusion paths, to avoid local overheating, or to attain an improved utilization of the catalyst volumes through a suitable internal structure of the catalysts. Individual measures of this kind so far have resulted in a situation where, with such catalysts used on an industrial scale, one can achieve only a comparatively unsatisfactory productivity per catalyst volume used during industrial production of acrolein. This is a matter of considerable disadvantage in economic terms because, to balance this out, one must use large and expensive reactors with a high filling volume for the catalyst to perform the reaction.
One of the objects of the present invention was to create a catalyst for making unsaturated aldehydes from olefins, particularly of acrolein from propene, through oxidation with a oxygen-containing gas. A catalyst is described which, with a large ratio between geometric surface and volume and simultaneously small low diffusion resistance, will overcome the disadvantages of the prior art and in particular will facilitate a surprisingly high productivity.
Another object of the invention was a manufacturing process for the described catalyst in terms of its basic composition and the further developed variants.